1. Corn
A variety of cereal grains and other plants are grown for use as food. Major cereal grains include corn, rice, wheat, barley, sorghum (milo), millets, oats, and rye. Other plants include potatoes, cassava, and artichokes. Corn is the most important cereal grain grown in the United States. Corn is sometimes called maize and has the scientific name Zea mays. A mature corn plant consists of a stalk with an ear of corn encased within a husk. The ear of corn consists of about 800 kernels on a cylindrical cob. The kernels are eaten whole and are also processed into a wide variety of food and industrial products. The other parts of the corn plant (i.e., the stalk, leaves, husk, and cob) are commonly used for animal feed, but are sometimes processed into a variety of food and industrial products.
In more detail, the corn kernel consist of three main parts: (1) the pericarp; (2) the endosperm; and (3) the germ. The pericarp (also known as the seed coat or bran) is the outer covering of the kernel. It consists primarily of relatively coarse fiber. The endosperm is the energy reserve for the plant. It consists primarily of starch, protein (also known as gluten), and small amounts of relatively fine fiber. The germ (also known as the embryo) consists primarily of oil and a miniature plant with a root-like portion and several embryonic leaves.
2. Starch
Starch is stored in a corn kernel in the form of discrete crystalline bodies known as granules. On a molecular level, starch is a polymer of anhydroglucose units (C6H10O5). Arnhydroglucose units combine with a water molecule (H2O) to produce the common sugar glucose (C6H12O6) so readily that starch is commonly referred to as a polymer of glucose. Starch is a member of the general class of carbohydrates known as polysaccharides. Polysaccharides contain multiple saccharide units (in contrast to disaccharides which contain two saccharide units and monosaccharides which contain a single saccharide unit). Polysaccharides made up of the same saccharide units (such as cellulose) are sometimes referred to as homopolysaccharides while polysaccharides made up of different saccharide units are sometimes referred to as heteropolysaccharides.
The length of a saccharide chain (the number of saccharide units in it) is sometimes described by stating its “degree of polymerization” (abbreviated to D.P.). Starch has a D.P. of 1000 or more. Maltose is a disaccharide (its D.P. is 2) that is composed of two glucose units. Glucose (also known as dextrose) is a monosaccharide (its D.P. is 1).
Saccharides having a D.P. of about 5 or less are sometimes referred to as sugars. Monosaccharide sugars containing six carbon atoms (e.g., glucose) are sometimes referred to as hexoses and sugars containing five carbon atoms are sometimes referred to as pentoses.
The anhydroglucose units in starch are connected to each other in one of two ways. When connected together in alpha-1,4-linkages, the starch molecule is linear. When connected together in alpha-1,6-linkages, a branch occurs. The relative number of the two linkages varies depending on the variety of corn. Both types of linkages are sometimes referred to as glucosidic linkages.
3. Fiber
As mentioned above, the pericarp and endosperm of the corn kernel contain fiber. The fiber comprises cellulose, hemicellulose, lignin, pectin, and relatively small amounts of other materials. Fiber is present in relatively small amounts in the corn kernel, but is present in much greater amounts in other corn components such as the cob, husk, leaves, and stalk. Fiber is also present in other plants. The combination of cellulose and lignin is sometimes known as lignocellulose and the combination of cellulose, lignin, and hemicellulose is sometimes known as lignocellulosic biomass. As used herein, the term “fiber” (and its alternative spelling “fibre”) refers to cellulose, hemicellulose, lignin, and pectin. Each of the components of fiber is discussed in detail below.
Cellulose, like starch, is a polymer of anhydroglucose units (C6H10O5). However, where the anhydroglucose units in starch are connected to each other in alpha-1,4 and alpha 1,6-linkages, the anhydroglucose units in cellulose are connected to each other in beta-1,4-linkages which give the cellulose molecule a linear, chain-like configuration. Cellulose is a rigid, crystalline structure because its molecules form attractions, known as hydrogen bonds, with adjoining molecules. Cellulose can be converted to glucose by breaking the beta-1,4-linkages by treatment with enzymes and/or by treatment at high temperatures and pressures in the presence of water.
The beta-1,4-linkages in cellulose are not broken down in the human digestive system. Accordingly, cellulose provides no nutritional benefit to humans and passes through the digestive system intact. Cellulose in the human diet is often referred to as fiber or roughage. In contrast to humans, some mammals are able to digest cellulose. Ruminant animals, such as cattle, sheep, goats, and deer, have certain types of bacteria in their digestive systems that produce enzymes that can break down the beta-1,4-linkages to free individual glucose molecules.
Hemicellulose is a heteropolysaccharide that, like cellulose, is present in the corn kernel and in the cell walls of other plants. On a molecular level, hemicellulose is a polymer of several pentose and hexose sugars, including xylose, mannose, galactose, arabinose, and glucose. Where cellulose molecules are linear and form a rigid crystalline structure, hemicellulose molecules are branched and form a much weaker structure.
Lignin is a complex compound composed of linked six-carbon phenolic rings that is present in the corn kernel and in the cell walls of other plants. After cellulose, lignin is the most abundant organic molecule on Earth. Lignin is a non-crystalline substance that acts as a binder of the cellulose in plants.
Pectin is heteropolysaccharide that is also present in cell walls. On a molecular level, pectin is a polymer of several compounds, including galacturonic acid, rhamnose, galactose, arabinose, and xylose.
4. Conventional Corn Refining Processes
A wide variety of processes have been used to separate the various components of corn. These separation processes are commonly known as corn refining. One of the processes is known as the dry milling process. In this process, the corn kernels are first cleaned and then soaked in water to increase their moisture content. The softened corn kernels are then ground in coarse mills to break the kernel into three basic types of pieces—pericarp, germ, and endosperm. The pieces are then screened to separate the relatively small pericarp and germ from the relatively large endosperm. The pericarp and the germ are then separated from each other. The germs are then dried and the oil is removed. The remaining germ is typically used for animal feed. The endosperm (containing most of the starch and protein from the kernel) is further processed in various ways. As described below, one of the ways is to convert the starch to glucose and then ferment the glucose to ethanol.
A second corn refining process is known as the wet milling process. In this process, the corn kernels are first cleaned and then steeped (soaked) in warm water containing sulfurous acid (H2SO3). During steeping, water soluble proteins and other substances dissolve into the steepwater. After steeping, the softened corn kernels are ground in coarse mills to break the kernel without damaging the germ. The kernels then flow to centrifugal separators which separate the less dense germs from the denser pericarp and endosperm. The germs are then dried and the oil is removed.
The pericarp and endosperm are then ground in fine mills. The finely ground stream flows to screens which separate the small particle size pericarp from the larger particle size endosperm. The endosperm stream then flows to centrifugal separators that separate the less dense protein from the denser starch. The finished starch is in granular form and is suitable for many different types of further processing.
For example, the starch can be dried and sold as unmodified starch. The starch can be modified and used for food or industrial purposes. The starch polymer can be partially hydrolyzed (i.e., shortened or reduced in D.P.) to produce corn syrup or hydrolyzed all the way to the individual glucose units. If completely hydrolyzed to glucose, the glucose molecules can be isomerized to fructose. Fructose is considerably sweeter than glucose and is widely used in the food industry. The starch can also be used for fermentation, as described in more detail below.
5. Fermentation
Fermentation is a process by which microorganisms such as yeast digest sugars to produce ethanol and carbon dioxide. The basic reaction isC6H12O6—2C2H5OH+2CO2 
Yeast reproduce aerobically (oxygen is required) but can conduct fermentation anaerobically (without oxygen). The fermented mixture (commonly known as the beer mash) is then distilled to recover the ethanol. Distillation is a process in which a liquid mixture is heated to vaporize the components having the highest vapor pressures (lowest boiling points). The vapors are then condensed to produce a liquid that is enriched in the more volatile compounds.
Various processes have been disclosed for producing fuel ethanol from corn. The processes all convert the starch present in the corn kernel into glucose which is then fermented. For example, one process uses the endosperm isolated from a dry milling process as the feed material. This process is illustrated in FIG. 1. Another process uses the starch isolated from the wet milling process as its feed material. This process is illustrated in FIG. 2. Conventional processes for producing ethanol from corn produce a maximum of about 2.6 to 2.8 gallons of fuel ethanol per bushel of corn. Conventional processes do not convert any of the lignocellulose materials in corn to sugars and, therefore, the lignocellulose does not contribute to ethanol production.
6. The Langhauser Process
An efficient process for producing ethanol from corn is disclosed in Langhauser, U.S. Patent Application Publication No. 2004/0187863, Sep. 30, 2004, and Langhauser, U.S. patent application Ser. No. 11/336,324, Jan. 20, 2006, now U.S. Pat. No. 7,452,425 B1, both of which are hereby incorporated by reference in their entireties. The Langhauser process is illustrated in simplified form in FIG. 3. More detailed schematics of the Langhauser process are shown in FIGS. 5 and 6. By recovering increased amounts of starch, the Langhauser process produces about 2.85 gallons of fuel ethanol per bushel of corn.
The Langhauser process produces a coarse fiber stream (consisting of cellulose from the pericarp and various other pericarp components including hemicellulose, lignin, pectin, and sugars) as a co-product. Langhauser discloses that the coarse fiber stream can be treated in various ways. It can be steam exploded, expanded, chemically treated, treated with cellulase enzymes, or used for fermentation. The stream can also be further separated to produce cellulose, hemicellulose, lignin, and pectin.
The Langhauser process also produces a fine fiber stream (consisting primarily of cellulose from the endosperm). Langhauser discloses that the fine fiber stream can be used for human dietary fiber, oil and sugar extraction, alcohol fermentation, animal feed blends, or can be blended with other products.
7. The Guffey Process
A process for producing ethanol and specialty chemicals from lignocellulosic materials such as wood, agricultural residues, and paper wastes is disclosed in “Fractionation of Lignocellulosic Biomass for Fuel-Grade Ethanol Production” by F. D. Guffey et al., Topical Report for Cooperative Agreement DE-FC26-98FT40323 for the U.S. Department of Energy, Western Research Institute, Laramie, Wyo., October 2002.
In the Guffey process, a lignocellulosic feed stream consisting of water insoluble cellulose, hemicellulose, lignin, and pectin is first treated under mild alkaline conditions to solubilize the hemicellulose. The insolubles are then separated and treated under severe alkaline conditions to solubilize everything but the cellulose. The insoluble cellulose is then separated and treated with enzymes to produce glucose. The glucose is then fermented to ethanol.
8. Current Economic Conditions
As the world population and demand for fuels both increase, there is an increased demand for both food and ethanol from corn. It is estimated that approximately one-third of the corn grown in the United States in 2008 will be refined into ethanol. Some experts believe that the use of corn for ethanol is contributing to increased food prices. Accordingly, there is a demand for a process that greatly increases the yield of ethanol from a bushel of corn.